Questions And Answers

When taking measurements from our LCD megohmeter and using the 5 lb. electrodes, we measured 1.8 x 10^8 Ohm/sq. on his dissipative table laminate. After inserting a foam disc of 1/4" height and 2" diameter (apparently impregnated with carbon, surface resistivity: 10^4 Ohm/sq.) between one of the electrodes and the table surface, we measured 3.4 x 10^7 Ohm/sq. Can you offer any explanation as to why surface resistivity would decrease upon inserting a conductive object between a dissipative surface and the electrode? It seems to counter common sense but it is the fact. - Anonymous, Born, Germany

Answer

There are two phenomena happening here. One is polarization. Every time you make a measurement, you drive a current between the probes and depending on the material and the potential (100 Volts) you may polarize the conductive elements (aligning them up…making the path between the probes even more conductive…..they may relax after a while though). Second is a possible increase to the surface contact. The rubber pad on the bottom of our 5 lb probes is less then 10 ohms, but not as soft as the foam. The foam, when compressed with a 5 lb probe, may make more surface contact (contact more of the 2” diameter surface) then just the rubber pad by itself. This in turn will decrease the contact resistance and yield a lower resistance reading.